1. Field of the Invention
The present invention relates to a method and an apparatus for producing a uranium foil with fine crystalline granules by forming the foil by the gravitational dropping of molten uranium or uranium alloy and then rapidly cooling the foil by the contact with cooling rolls, and a foil produced thereby.
More particularly, the present invention relates to a method and an apparatus for easily producing a high-purity and high-quality uranium foil having a fine isotropic structure without requiring hot rolling and heat treatment processes, in which the surface of the foil is prevented from oxidizing and residual stress is not imparted to the foil, and a foil produced thereby, thus improving the productivity and the economic efficiency of the production process.
2. Description of the Related Art
Known in the art are several methods and apparatus for producing a uranium foil, as follows.
U.S. Pat. No. 3,010,890 discloses a method for producing uranium alloy with fine particles by alpha-annealing and beta-quenching. Since the uranium alloy is produced by heat treatment and rolling processes, such a method has a problem of imparting residual stress.
The method disclosed by U.S. Pat. No. 3,285,737 also employs heat treatment and rolling processes in alpha-annealing and beta-quenching, thereby having the same problem of imparting residual stress.
U.S. Pat. No. 3,888,300 discloses an apparatus for continuously casting metals and metal alloys under the vacuum condition, in which rolls are located within a suction chamber separated by diaphragm walls, and molten metal is guided and discharged into the suction chamber under the proper vacuum state.
U.S. Pat. No. 3,969,160 discloses a high-strength ductile uranium alloy consisting titanium, vanadium, and uranium, which possess desirable ductility while retaining the anti-corrosion characteristics of titanium.
U.S. Pat. No. 4,154,283 discloses a process for producing metal alloy noncrystalline filaments having improved surface characteristics and enhanced mechanical properties using a quenching wheel in a partial vacuum.
U.S. Pat. No. 4,577,081 discloses a method and an apparatus for heating a billet of nonmagnetic metal material to a forging temperature and reheating the billet using an inductive heating coil.
U.S. Pat. No. 4,714,104 discloses an apparatus for continuously casting a metal, in which the metal is degassed under vacuum, thereby preventing the fluctuation of molten metal at the surface of the metal.
U.S. Pat. No. 4,982,780 discloses a method for producing a noncrystalline metal filament with a uniform thickness, in which a width of the filament is varied by the rotational directions of a chill.
U.S. Pat. No. 5,720,336 discloses a method for continuously casting a metal strip, in which a casting pool is created above a pair of parallel casting rolls engaged with each other, and molten metal is fed into the nip between the casting rolls.
U.S. Pat. No. 5,960,856 discloses a method and an apparatus for casting a metal strip including iron, in which a casting pool of molten metal is supported on a pair of casting rolls, the molten metal is cast into the strop by moving downward from a nip between the casting rolls, and the cast metal strip is completely cooled by means of non-contact heat absorbers.
Further, in a method for producing a uranium foil known to the skilled in the art, an ingot is made of uranium or uranium alloy, cut, and then fed through the hot rolling process, thereby being formed into the foil.
More specifically, the ingot is maintained at a constant temperature of 1,300° C. and then cast into a sheet in a vacuum inductive melting furnace. Otherwise, the ingot is cut into sheets with a proper size, and then the cut sheets repeatedly go through hot rolling and heat treatment processes at a temperature of 600° C. under the inert gas atmosphere so that the thickness of the sheet is gradually reduced. Finally, a uranium foil with a thickness of 100 μm to 500 μm is produced.
In order to prevent the swelling of the uranium foil during the irritation test, an isotropic structure of the foil having fine crystalline granules of the foil is required. Such isotropic structure of the foil is obtained by the heating process at 800° C. and subsequently the quenching process.
Therefore, the conventional method for producing the uranium foil is very complicated and troublesome.
Moreover, since the uranium or uranium alloy retains rigidity while lacking ductility, the hot rolling of the uranium or uranium alloy is very difficult.
During the rolling process, the residual stress existing in the uranium causes cracks in the foil, thereby producing defective foils and reducing the recovery rate of the uranium.
Therefore, the conventional method for producing the uranium foil with the reduced recovery rate is noneconomical.
Since uranium is an easily oxidizable material, the uranium must go through the hot rolling process under a vacuum condition or an inert gas atmosphere. Accordingly, the repetition of the hot rolling processes of the uranium is very troublesome, requires a long period of time, and remarkably reduces the productivity of the uranium foil.
The produced uranium foil having residual stress due to the repetition of the hot rolling process may be deformed or damaged due to such thermal cycling during the production or the irradiation.
The method for producing uranium foil by the hot rolling process further requires an additional process for removing impurities such as a surface-oxidized product mixed at the rolling process, thereby being complicated.